Dielectric constant is defined as a measure of the ability of a dielectric to store an electric charge. A dielectric is a nonconducting substance or an insulator. Dielectric constant is directly proportional to capacitance of a material, which means that capacitance is reduced if dielectric constant of a material is reduced. For high frequency high-speed digital circuits, capacitance of substrates and coatings are critical to reliable functioning of the circuits. Present computer operations are limited by the coupling capacitance between circuit paths and integrated circuits on multilayer boards. The computing speed between integrated circuits is reduced by this capacitance and the power required to operate them is increased.
With recent trends toward microminiaturization and utilization of very thin conductor lines, close spacings, and very thin insulation of 5 mils or less, greater demands are being placed on the insulating layer. Insulating materials must possess very low dielectric constants and at the same time must retain other required engineering and manufacturing properties. For high frequency linear circuits, such as those used in radar assemblies, the dielectric constant of insulators again becomes important, especially since it may vary with changes in frequency.
Poly(tetrafluoroethylene), which is a solid at room temperature and is known as Teflon.RTM. dielectric material, has dielectric constant in the range of 2.00-2.08 and tetrafluoroethylene which is a gas at room temperature, has dielectric constant in the range of 1.89-1.93. Poly(tetrafluoroethylene) is completely chemically inert, has excellent electrical properties, has outstanding stability, and retains mechanical properties at high temperatures. The problem with poly(tetrafluoroethylene) is that it is not processable Teflon.RTM. AF dielectric material, a commercial material, is believed to be a terpolymer of tetrafluoroethylene, perfluoropropylene and a derivative of hexafluoracetone. Teflon AF material is believed to have a dielectric constant of 1.92 and although it is more processable than poly(tetrafluoroethylene), it still lacks adequate processability.
U.S. Pat. Nos. 4,452,998 and 4,356,296 disclose fluorinated diacrylic esters which are prepared by reacting a substituted benzene with acryloyl chloride in a highly fluorinated solvent in the presence of a tertiary amine, such as triethyl amine, or another acid acceptor, at 5.degree. to 20.degree. C. The monomer is purified and can then be polymerized. The fluorinated diacrylic esters of this patent are disclosed to be useful particularly in dental and biomedical applications.
The conventional material for ordinary printed circuit boards is epoxy resin impregnated glass fiber laminates although paper laminates may still be used in less arduous environments, such as in hi-fi equipment and domestic appliances. The use of flame retardant materials has become almost universal use to danger of fire in electronic and electrical equipment. The metal foils, preferably copper, applied to the non-conducting layers of a circuit board are usually 25 to 75 microns in thickness although some are 5 to 12 microns.
Multilayer printed circuit boards have been made of organic polymers, such as glass epoxy resins, since such materials have a low dielectric constant of about 4, however, it is impossible to directly mount integrated circuit chips in such boards because of their poor thermal resistance and inadequate physical properties to produce many insulated internal layers. This brought forth alumina ceramic circuit boards but problems were encountered due to the high dielectric constants of 9 to 10. With the advent of super computers, there is a tendency to use pulses with high frequency. At high frequency, the delay of propagation of signals is increased due to the relatively high dielectric constant of the insulating material. Also, there is an increase in electrostatic capacitance between adjacent wiring conductors which results in attenuation of signals as well as decrease in circuit impedance, if high dielectric constant insulator materials are used for insulators.
U.S. Pat. Nos. 4,452,998 and 4,356,296 to Griffith et al. disclose fluorinated diacrylic esters useful in dental and in biomedical applications as well as in the traditional acrylic applications such as coating, casting, encapsulanting, caulking, and the like.